1. Field of the Invention
The present invention relates to a Synthetic Aperture Radar (SAR) System capable of detecting moving targets. The system has no requirement as to directivity or fractional bandwidth. It is therefore possible to use it with an Ultra Wide Band (UWB) SAR system and Wide Beam (WB) transmission and reception. A UWB-WB SAR at low frequencies will add the capability to detect targets moving in forested areas, and at microwave frequencies it will give the capability of high resolution images of the moving target.
2. Description of the Related Art
Today there only exists UWB-WB systems at low frequencies, but in time there will be systems also at higher frequencies. The low frequency UWB-WB SAR system has shown its effectiveness to detect concealed targets. This unique capability is a result of the low frequencies in combination with the relatively high resolution that a UWB-WB SAR sensor gives. This has been successfully demonstrated for instance in the CARABAS(trademark) system, Swedish patent 8406007-8 (456 117) and U.S. Pat. Nos. 4,866,446 and 4,965,582 all incorporated by reference herein. The resolution in the CARABAS(trademark) system is smaller than the center frequency wavelength. To reach this high resolution a very large integration time is needed which demands good motion compensation. Fourier-domain techniques do not adapt to this problem very well, so resolution is reached by time domain backprojection. Fast backprojection techniques (a domain of methods), such as local backprojection (LBP), described i.a. in the Swedish public patent application 9503275-1 and U.S. Pat. No. 5,969,662 both incorporated by reference herein, and factorized backprojection (FBP), described in xe2x80x9cL. M. H. Ulander, H. Hellsten, G. Stenstrxc3x6m: Synthetic Aperture Radar Processing Using Fast Factorised Backprojection, Proc. of EUSAR 2000, 3rd European Conference on Synthetic Aperture Radar, Germany, pp. 753-756, which is incorporated by reference herein, are approximate and much faster then the global backprojection (GBP).
In UWB-WB SAR only one scatter can appear in the resolution cell and therefore no speckle noise is seen in the images. In particular at low frequencies the radar signal will be stable. Objects that cause radar reflection have a physical size of the wavelength and larger. Low frequency scatters are meters in size, and these large objects do not move between the occasions. As an example, in forests it is not the leaves or branches which cause the reflection, but rather the stable ground-trunk that is the major backscatter contributor.
Detection of moving targets requires maximization of the target signal compared to the clutter signal. In order to filter the strong clutter signal from stationary objects, the displaced-phase-center-antenna (DPCA) method was developed. This technique needs strict spatial alignment and system stability. In the extension of adaptive antenna technique the space-time adaptive processing (STAP) was found. The STAP is not only adaptive, in the space-time two-dimensional space the clutter spectrum is basically a narrow ridge, so that slow moving targets can be detected.
Clutter suppression by GMTI filtering has in latest years developed in combination with SAR. In a SAR GMTI system the moving target will not only be detected, but also imaged to high resolution in its surroundings. Movement of a target will influence the focusing in the SAR process of the moving target compared to its surroundings. The moving target will be smeared and shifted in location. For SAR with a side-looking narrow beam antenna system these effects are known, and methods for detection of slow moving targets has also been proposed.
In the latest years there has been ongoing development to detect, focus and evaluate velocity components of a moving target in a SAR system. The idea is to use multi-channel antenna arrays to suppress the clutter signal from the stationary objects. The main detection scheme is to compare the likelihood ratio test with a threshold. Later experimental results have shown that SAR GMTI is a strong tool to detect and image moving targets in its surrounding. However, even in these narrow beam microwave systems, compensation in Doppler frequency is needed. In one known system the test statistics are first filtered for each Doppler frequency, and in another system antenna pattern was compensated in the Doppler domain.
The present invention gives a solution to the problem of detecting moving targets irrespective of fractional bandwidth and antenna directivity, by being designed using a synthetic aperture radar (SAR) system capable of detecting moving targets. The SAR system comprises a platform which moves over a number, which can be very large, of objects, e.g., in the form of a ground surface, and supports radar equipment which reproduces the objects by means of a fast backprojection synthetic aperture technique via at least two antennas, without requirement as to directivity or fractional bandwidth. According to the system, a signal-processing device operates by dividing the imaging process into three steps which are carried out in a determined order. The steps and the order are formation of sub-aperture beams at one speed, performing of clutter suppression, i.e., GMTI filtering, and detection of moving targets.